An active braking device is widely used to control a joint angular velocity. However, the active braking device has problems in which the system is complex and a complex controller is additionally required.
In contrast, a passive braking device is superior to the active braking device in terms of weight, compactness, energy efficiency, and responsiveness. Further, since the passive braking device is inherently safe, a complex fail-safe framework is not required. Generally, the passive braking device using a centrifugal force is used in a seat belt, an elevator, a fishing rod, and the like.
Since functions of most passive braking devices are affected by a direction of gravity, it is hard to apply the passive braking device to robots with a high degree of freedom. Further, since mechanism of the conventional passive braking device is heavy and bulky, and braking torque thereof is applied only in one direction, the passive braking device has a problem in being applied to a multi-body dynamic system. In addition, since a spring structure of the passive braking device is not completely independent with respect to a direction of gravity, the performance of the passive braking device is affected by gravity when used in a joint of the robot, and thus the development of a passive braking device having a structure capable of compensating for gravity is needed.